The present invention relates to a novel holographic photopolymer based on prepolymer-based polyurethane compositions, the preparation of said photopolymer and its use for a very wide range of optical applications.
Holographic media can be used, inter alia, as a data store, in the area of security technology (for example of the three-dimensional representation of persons or objects and the authentification of a person or of an article), the representation of objects, use as an advertising medium, use as auxiliary materials for the production of complex three-dimensional structures and as a screen or as components of screen parts and for the production of an optical element having the function of a lens, of a mirror, of a filter, of a diffusion screen, of a diffraction element, of a light conductor and/or of a mask.
Holographic media based on a polymer matrix and one or more compounds embedded therein and capable of radical polymerization, as writing monomers, are described, for example, in U.S. Pat. No. 6,743,552.
U.S. Pat. No. 6,743,552, U.S. Pat. No. 6,765,061, U.S. Pat. No. 6,780,546 and US 2006/0194120 disclose such two-component polyurethane formulations. Some of the formulations described there contain prepolymers as an isocyanate component, these being exclusively prepolymers having secondary isocyanate groups and therefore being unsatisfactory in the curing rate.
Polymer formulations for the production of holographic media were described, inter alia, in the non-prior-published patent applications EP 08017279.4, EP 08017277.8, EP 08017273.7, EP 08017275.2. EP 08017277.8 and EP 08017273.7 describe polyether-based and prepolymer-based polyurethane compositions which in general are suitable for the production of holographic media. EP 08017275.2 described polyurethane formulations comprising specific acrylates, which are suitable for recording holograms. Furthermore, EP 08017279.4 described for the first time a typical film structure and the application of various polyurethane formulations as a photopolymer in the film composite. The use of prepolymer-based polyurethane formulations for a coated film with the combination of a dye/initiator mixture which covers the complete visible wavelength spectrum (400-800 nm) has on the other hand not been previously described.
It was an object of the present invention to provide polyurethane compositions from which it is possible to prepare holographic photopolymers having good surface quality and processability, which also have a good refractive index contrast.